1. Field of the Invention
This invention relates to transponder systems in which an RF signal excites a transponder to transmit a responsive signal, and more particularly to transponder systems that communicate through barriers to RF transmission.
2. Description of the Related Art
RF transponder systems are commonly used to identify an object associated with the transponder, and/or to communicate various types of information about the object back to a receiving station. Such systems typically employ an exciter/reader that transmits an RF excitation signal, and a transponder that is energized by the excitation signal to transmit an identification code or other information back to the exciter/reader. Transponder cards are also used to protect clothing and other merchandise from theft, in which case the mere presence of the object rather than its particular identification is transmitted back.
The transponders typically use a single antenna coil to both receive the excitation signal, and transmit back an information signal. The exciter/reader typically also uses a single antenna to both transmit the excitation signal and receive the information signal, although these functions can be separated if desired. A known transponder system is described in U.S. Pat. No. 4,730,188 to Milheiser.
The situation is complicated if the exciter/reader and transponder are separated by a barrier to RF transmission. For example, transponders with appropriate identifying codes can be positioned inside metal containers such as gas and air bottles, milk cans and chemical containers. These containers would normally block any attempt to transmit an RF signal to a transponder on the inside, or to transmit back an identification code. Transponders can also be used for machine tool identification in connection with computer numerical control machines, such as milling machines, that roboticly select among different tools. A transponder with a unique identification code is positioned internally within each tool to make sure the machine picks up the proper tool.
RF communication with a transponder within an RF-blocking enclosure has previously been accomplished by providing three separate coils: one mounted on the outside of the enclosure, a second on the inner surface of the enclosure wall, and the transponder coil itself within the interior of the enclosure. The coils mounted on the inside and outside of the enclosure wall are connected by a wire that extends through an opening in the wall. RF signals from an exciter/reader are received by the outside coil, transmitted along the wire through the opening to the second coil, and radiated from the second coil through an air gap to the final transponder coil; the return signal back to the exciter/reader follows the same path but in the opposite direction.
The additional coils required by this approach are subject to physical abuse, particularly the coil that is mounted outside the enclosure. The interconnecting wires between the outside and inside coils must also be protected. In addition, the extra coils are relatively large in size and can require that extra care be exercised when handling the enclosure.